Which version of Solr are you on?
Are you using SolrCloud or any distributed search?
In that case, I think( as already mentioned by Shawn) this could be related
[1] .
if it is just plain Solr, my shot in the dark is your boost function :
{!boost+b=recip(ms(NOW,field1),3.16e-11,1,1)}{!boost+b=recip(ms(NOW,field2),3.16e-11,1,1)}
I see you use NOW ( which changes continuosly).
it is normally suggested to round it ( for example NOW/HOUR or NOW/DAY).
The rounding granularity depends on the use case.
Time passing should not bring any change in ranking ( but it brings change
in the score).
I can imagine that if for any reason of rounding the score, we end up in
having different documents with the same score, then the internal ordinal
will be used for ranking them, bringing slightly different rankings.
This is very unlikely, but if we are using a single Solr, it's the first
thing that jumps to my mind.
[1] https://issues.apache.org/jira/browse/SOLR-5821
[2] https://github.com/fguery/lucene-solr/tree/replicaChoice
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Alessandro Benedetti
Search Consultant, R&D Software Engineer, Director
Sease Ltd. - www.sease.io
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